Fluoroelastomers (or fluororubbers) are coated on or impregnated in various substrates such as fabrics, fibers, metals, plastics, rubbers, etc. and widely used as industrial materials by making use of their good heat resistance, oil resistance, solvent resistance and chemical resistance. In particular, in the field of engine gaskets, recently fluoroelastomer-coated metal gaskets have been used as substitutes for asbestos gaskets, and it is expected that the fluoroelastomer-coated metal gaskets will be increasingly used.
Coating materials for metal gaskets should not be eroded with gas fluids under high temperature and high pressure conditions, have sufficient elasticity and flexibility at various temperatures and pressures, and maintain clamping forces necessary for preventing the leakage of the fluids from mated surfaces. In addition, such materials should have satisfactory resistances to antifreezes and engine oils.
As fluoroelastomer coating compositions for coating metal gaskets, polyol-curing coating compositions providing good sealing properties are usually used. In such a case, a primer should be applied to a substrate for adhering the coating composition to the substrate, and thus two steps should be carried out to form the primer layer and to form the fluoroelastomer layer.
On observing the degradation of a coating film under compression, the cured rubber of a polyol-curing fluoroelastomer composition more easily flows than that of a polyamine-curing fluoroelastomer composition when it is compressed at high temperatures for a long time, since no double bond contributes to the crosslinking in the former cured rubber.
When a polyamine-curing coating composition in which a primer component can be internally added to a curing agent is used, a coating process can be simplified, but a compression set, which is the most important property of a sealing material, is larger than a polyol-curing coating composition, and thus the clamping force should be increased to prevent the leakage from the mated surfaces.